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About three years ago, engineer David Hopstock was mulling ways to boost the sagging fortunes of Minnesota's Iron Range. The Roseville-based consultant had no ideas about how to improve the market for taconite pellets, the range's main export. But that wasn't what interested him. He was looking for potential uses for taconite tailings, the waste material left behind in staggering quantities by taconite production.
Hopstock, who has a Ph.D. in metallurgical engineering, knew that the tailings contained a high percentage of a mineral called magnetite. From his days working at the U.S. Bureau of Mines, he also recalled that magnetite is an exceptionally good absorber of microwave radiation. That's what triggered his eureka moment.
If the tailings were mixed with asphalt, Hopstock speculates, a powerful microwave generator might then be applied to the road surface. That could have a handful of practical applications. In cold, icy weather, for instance, a microwave-emitting device hitched to a plow truck could be used to deice the road, thereby eliminating the need for salt or other corrosive and environmentally hazardous chemical treatments. Even if it proved impractical or too expensive for large stretches of road, he says, it might be very useful for bridge decks and runways.
Because taconite tailings are easily heated with a microwave, Hopstock theorizes, a pothole patch made with tailings would work much better than the current "throw and go" pothole repair technique. And in any weather, Hopstock thinks, the paving of new roads might be vastly improved through the use of a microwave, since it is critical that asphalt temperatures be maintained within certain parameters to set properly.
Will it work? Hopstock has yet to conduct a field trial, because he hasn't been able to get adequate funding. But his small-scale experiments--kitchen scale, in fact--have yielded some promising results. In one such trial, Hopstock filled a Styrofoam cup with magnetite-rich epoxy, then covered it with water and put it in a freezer. After a hard layer of ice formed over the epoxy, Hopstock placed the container in a standard-sized microwave oven and zapped it. Within a matter of seconds, he noted, the epoxy had absorbed enough heat that the ice ceased to adhere to the surface. "I think that demonstrated, well, that it's not a totally wacky idea," Hopstock says with a laugh.
With such results in hand, dramatized by a detailed Power Point presentation, Hopstock and research colleague Larry Zanko floated their proposed technology at a seminar hosted by the University of Minnesota's Center for Transportation Studies. Erland Lukanen, the director of the Pavement Research Institute at the U, says he was intrigued by the potential, especially the pothole patching application.
The chief issue, Lukanen says, is economic. It's not just an issue of equipping highway trucks with giant microwaves. Because bulk materials are expensive to transport, he explains, most aggregate comes from local quarries. For any projects outside the Iron Range, that raises the question: Would any city or state agency be willing to pay the extra freight to haul taconite tailings from northeast Minnesota? Probably not, in the near future. But as the Twin Cities metro area continues to grow, Lukanen notes, it has become more difficult to develop new quarries, which may mean the economic equation will shift.
Hopstock and Zanko's proposal for the tailings is not without critics. At the Center for Transportation Studies seminar, two old lions of Minnesota's environmental movement--retired federal judge Miles Lord and former Minnesota Pollution Control Agency chief Grant Merritt--raised objections based on the possible health risks. Both Lord and Merritt are concerned because some taconite tailings are known to contain asbestos and asbestos-like fibers. Those fibers, they fear, may be related to the elevated levels of a rare but deadly cancer called mesothelioma on the Iron Range.
From 1988 to 1999, according to a 2003 Minnesota Department of Health study, there were 81 identified cases of mesothelioma among residents of northeastern Minnesota, nearly double the expected rate. For that reason alone, Merritt contends, any use of taconite tailings in highways should be subjected to a full environmental review.
Lord, who issued the landmark 1974 ruling that prohibited the dumping of taconite tailings into Lake Superior, takes a more conspiratorial view. The use of taconite tailings on highways, Lord contends, represents "a deliberate attempt by the taconite industry and its owners, 'Big Steel,' to spread deadly particles on Minnesota highways, thereby making it difficult, if not impossible, to pinpoint the cause of cancer deaths on the Iron Range because cancer will occur throughout the state of Minnesota."
In Zanko's view, such claims are both needlessly inflammatory and "pretty much a red herring." He points out that the Department of Health study concluded that higher levels of mesothelioma on the Range were most likely the result of exposure to commercial asbestos, not taconite dust. A fellow at the University of Minnesota-Duluth's Natural Resources Research Institute, Zanko further notes that tailings have been used in road construction in northeast Minnesota for decades, without apparent ill effect. (And, he adds, highway engineers like using the tailings because they are very durable and have superior friction characteristics).